1. Field of the Invention
The present invention relates generally to a mobile communication system. More particularly, the present invention relates to a method and apparatus for transmitting and receiving a status report indicating a received status of packet data.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd generation asynchronous mobile communication system using Wideband Code Division Multiple Access (WCDMA) based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), both of which are European mobile communication systems.
The general UMTS system is composed of a Core Network (CN) and a plurality of Radio Network Subsystems (RNSs). The RNSs constitute a UMTS Terrestrial Radio Access Network (UTRAN). The CN includes a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) in order to connect the UTRAN to a packet data network such as the Internet.
The RNS is composed of Radio Network Controllers (RNCs) and a plurality of Node Bs. Each RNC is connected to its associated Node Bs via an interface called Iub. A connection between the RNCs is made by an interface called Iur. In addition, a connection between a User Equipment (UE) and the UTRAN is made by an interface called Uu. The RNC allocates radio resources for its own Node Bs and the Node Bs actually provide the radio resources allocated from the RNC, to the UE. The radio resources are configured separately for each individual cell, and the radio resources provided by each Node B mean the radio resources associated with a particular cell is managed by the corresponding Node B. The UE sets up a radio channel using the radio resources associated with the particular cell managed by the Node B, and transmits and receives data over a set radio channel.
Meanwhile, in 3rd Generation Partnership Project (3GPP) in charge of UMTS standardization, Long Term Evolution (LTE) of the UMTS system is now under discussion. LTE is a technology for implementing communication based on high-speed packets of about 100 Mbps, aiming at deployment in around 2010. Accordingly, several plans are under discussion. For example, there is one plan to reduce the number of nodes located in a transmission path by simplifying a configuration of the network, and another plan to approach radio protocols as close to radio channels as possible. As a result, the LTE configuration is expected to change from a 4-node configuration of the existing UMTS system to a 2-node or 3-node configuration.
FIG. 1 illustrates an exemplary configuration of an Evolved UMTS mobile communication system. As illustrated, Evolved Radio Access Networks (E-RANs) 110 and 112 are simplified to a 2-node configuration of Evolved Node Bs (ENBs) 120, 122, 124, 126 and 128), and Evolved Gateway GPRS Serving Node (EGGSNs) 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (EP) network 114 via the E-RANs 110 and 112.
The ENBs 120 to 128 correspond to legacy Node Bs of the UMTS system, and are connected to the UE 101 via radio channels. Compared with the legacy Node Bs, the ENBs 120 to 128 perform complex functions. In LTE, all user traffic including real-time service such as Voice over IP (VoIP) are serviced through a shared channel, so there is no need for an apparatus for collecting status information of UEs and performing scheduling thereon. For example, the ENBs 120 to 128 take charge of the scheduling.
Like High Speed Downlink Packet Access (HSDPA) or Enhanced uplink Dedicated Channel (E-DCH) supported by the UMTS system, LTE also performs Hybrid Automatic Retransmission Request (HARQ) between the ENBs 120 to 128 and the UE 101. HARQ refers to a technique for soft-combining previously received data with retransmitted data without discarding previously received data, thereby increasing a reception success rate. However, because it is not possible to satisfy various Quality-of-Service (QoS) requirements only with HARQ, outer Automatic Retransmission Request (ARQ) can be performed in an upper layer, and the outer ARQ is also performed between the UE 101 and the ENBs 120 to 128.
In order to implement a data rate of a maximum of 100 Mbps, LTE can use Orthogonal Frequency Division Multiplexing (OFDM) as a radio access technology in the 20-MHz bandwidth. In addition, an Adaptive Modulation & Coding (AMC) scheme for determining a modulation scheme and a channel coding rate according to a channel status of the UE can be applied to LTE.
Meanwhile, radio communication includes a process of segmenting or concatenating packets having various sizes, generated in an application layer, in an appropriate size, attaching a header containing necessary additional information thereto, and transmitting the header-attached segments through a radio channel. The operation of matching the size of the packets to the size appropriate for transmission and reception over the radio channel is called ‘framing’.
The typical UMTS communication system frames transmission data once in a Radio Link Control (RLC) layer, and then performs framing again in a Medium Access Control (MAC) layer. This is because although the UMTS communication is designed based on dedicated channels that operate only with RLC framing, there is a need for framing of the MAC layer due to introduction of the shared channel of HSDPA/E-DCH. More specifically, the RLC layer performs an operation of matching a packet size of an upper layer to a predetermined size, and the MAC layer performs an operation of concatenating the packets provided from the RLC layer according to the amount of the packets to be transmitted in the next Transmission Time Interval (TTI). In IP communication, the upper layer can be an IP layer.
FIGS. 2A and 2B illustrate a hierarchical structure for framing upper layer data, and a frame format thereof in a typical UMTS communication system, respectively.
Referring to FIG. 2A, a UMTS radio protocol is composed of a physical layer 275, a MAC layer 270, and RLC layers 260 and 265. If data (for example, an IP packet) generated in upper layers 250 and 255 is delivered to any one of the RLC entities 260 and 265, the RLC entity 260 or 265 segments the IP packet according to a predetermined size, and attaches a header to each of the segmented parts, thereby generating a RLC Protocol Data Unit (PDU). As illustrated in FIG. 2B, the header includes a Sequence Number (SN) 210, an E bit 215 and a Length Indicator (LI) 220. The LI 220, information indicating an end position of the IP packet, is optional header information inserted when the last part of the IP packet is included in the RLC PDU. For example, because a RLC PDU 205 includes the last part of an IP packet 204, a LI is inserted in the RLC PDU 205.
The MAC layer 270 receives RLC PDUs from the RLC layers 260 and 265, and concatenates the RLC PDUs according to the size of a MAC PDU 206. In a system to which Node B scheduling is applied, like HSDPA or E-DCH, a size of the MAC PDU 206 is variable according to the amount of scheduled transmission resources. Therefore, a Size Index (SID) 208 indicating a size of each RLC PDU, and an N bit 209 indicating the number of contained RLC PDUs are inserted in the MAC PDU 206 as header information. RLC PDUs delivered from more than two RLC entities can be concatenated to one MAC PDU 206. In order to identify the RLC PDUs, a Multiplexing Identifier (MID) 211 is inserted in the head of the MAC PDU 206. The multiplexing information 211 indicates to which RLC entities the contained RLC PDUs should be delivered.
In HARQ operation, received packets may suffer sequence turnover, and in order to solve the sequence turnover problem, a receiver receiving the MAC PDU 206 may need a separate SN. Therefore, a Transmission Sequence Number (TSN) 207 is assigned for the RLC PDUs generated in one RLC entity among the RLC PDUs contained in the MAC PDU 206 in a concatenated fashion. That is, the RLC PDUs 202 and 203 generated by different RLC entities have different TSN values.
As described in FIGS. 2A and 2B, the overlapping framing scheme used in the typical UMTS network is disadvantageous because overlapped information is used in the header. For example, the SN 210 is inserted in each RLC PDU, and the TSN 207 is inserted in the MAC PDU 206. In addition, as the SN 210 is inserted in each RLC PDU having a smaller size, overhead increases undesirably.
Accordingly, there is a need for an improved method and apparatus for transmitting and receiving a status report that comprises a received status of packet data.